This invention relates to improvements in acoustoelectric transducers in the form of an array of piezoelectric crystals mounted in parallel on an energy absorbing base. A prior art transducer of this type is schematically illustrated in FIG. 1 wherein crystals T.sub.3, T.sub.4 and T.sub.5 are shown mounted on a base 8. Transducers of this type are used in ultrasonic imaging apparatus to transmit pulses of a few cycles of acoustic waves into the body under examination and to convert acoustic energy reflected back to them by internal structures in the body into corresponding electrical signals. The signals are summed and applied to control the intensity of the image. Although the means for exciting the crystals so that they will transmit the pulses of acoustic waves are not shown in FIG. 1, they function by applying a few cycles of an alternating voltage across the thickness of the crystal, i.e., between the base and the opposite side, the cycle having a frequency equal to the frequency at which the crystal resonates in thickness. The pulses are made as short as possible in order to optimize the range resolution.
It has been found, however, that excitation of a crystal in a thickness mode causes surface waves having a frequency less than the resonant frequency of the thickness mode to emanate in opposite directions along the top of the base 8 as indicated by the arrows 10 and 12, emanating from the crystal T.sub.4 (see Page 8 and FIG. 4d of "Rayleigh and Lamb Waves" by I. A. Viktorow, Plenum Press 1967). When the surface waves reach the crystals T.sub.3 and T.sub.5 respectively, they are reflected with a change in phase depending on the impedance of the crystals back toward the crystal T.sub.4 as indicated by the arrows 10' and 12'. Because the total distance of the paths along the arrows 10 and 10' is the same as the total distance along the paths 12 and 12', the phase shift caused by the distance traversed is the same so that the reflected waves return to the crystal T.sub.4 with identical phases and create a resonant condition which induces crystal T.sub.4 to manifest the lower frequency resonant mode. The mode conversion from the surface to bulk mode by the induced resonance effectively prolongs the fundamental thickness mode of the crystal T.sub.4 so that the transmitted acoustic pulses are broadened, thereby reducing the range resolution that might otherwise be obtained. Inherent in the conversion from surface to bulk modes is an element of delay caused by the transit time so that the excitation of the crystal T.sub.4 responds to the main excitation and to delayed additions that tend to broaden the pulse.